This invention relates to a means of minimizing the tribo-electrification of conveyed particles in a pneumatic conveying system.
In most pneumatic conveying systems, electrostatic charges are produced by the known phenomenon of tribo-electrification, generated between the conveyed particles and the pipeline walls. In certain systems, charged particles could lead to safety hazards or processing problems, and accordingly it is highly desirable to reduce charging.
Whether the pipelines are made out of metal or plastic materials, the tribo-electrification of particles continues indefinitely. Metallic pipelines, when connected to ground, ensure the complete discharge of the tribocharge developed on the interior surfaces. However, in the case of plastic pipelines, the charges on the inside surfaces could reach very high values. In a cylindrical geometry, unless the electric charges on the inside surfaces of the pipe relax sufficiently to maintain a steady state, the surface charges continue to build up, leading either to eventual breakdown through the pipe wall or to spark discharges along the inside surfaces to the nearest ground.
Tribo-electrification has the following known characteristics:
1. A flat metallic plate isolated from the ground, when continuously bombarded by particles of a different work function, will gradually acquire a charge which reaches a maximum depending on the electric field which is generated at the surface of the plate.
2. A dielectric plane surface will behave similarly to a metallic plate. When the surface electric field of the dielectric reaches a certain value, the tribo-electrification stops and the surface charge is maintained at that maximum value.
3. A long cylindrical dielectric surface will continue indefinitely to acquire surface charges which eventually will lead to breakdown. The reason for that is that the electric field on the inside surface of a long cylinder is zero and hence it does not impede the tribo-electrification.